154 MUTATION. 



quite impossible to know whether a new dominant form, spon- 

 taneously originated, thanked its origin to the acquisition of 

 a new gene or to the loss of one which had been present. But 

 we have seen, that we have no reason to assume that the loss 

 of a gene could produce a form, dominant over that from 

 which it was derived. And this is further strengthened by the 

 observation, that as yet the only mutations which have been 

 verified in a sufficient way, have, with a few significant ex- 

 ceptions, always resulted in the production of a recessive form. 

 As I will later show, we may not conclude that new genes have 

 been acquired spontaneously in a species, because we know of 

 the existence of tame cultivated individuals, which possess 

 genes which the wild parent-form has not. In this connection it 

 is very significant to observe the fact, to which Bateson 

 draws attention in his "Problems of Genetics," namely that 

 plants which, as the Sweet-pea, cannot possibly be crossed 

 with other wild species, and which have given rise to hundreds 

 upon hundreds of distinct domestic species, have not produced 

 dominant forms. 



Lotsy goes so far as to deny that spontaneous change of 

 genotype, real cases of mutation, exist. It is true that it is very 

 difficult to obtain satisfactory proof, that a real mutation is 

 witnessed. We know of no case in the literature on the sub- 

 ject, in which it was probable that a new gene was spontan- 

 eously acquired. We may therefore simply ask the question : 

 What is necessary to prove that the production of a new form 

 is dueito a really spontaneous loss of one or more genes? It is 

 obvious that the occurrence, which can most easily be mis- 

 taken for mutation, is the production of an individual lacking 

 a gene, from a parent, or a pair of parents, impure, heterozy- 

 gous, for the gene in question. When a new form is f oimd wild, 

 and it is found that individuals of this new form breed true, it 

 'should not be allowable to speak of mutation. Formerly this 

 mistake was made more often than at present, for instance by 

 Blaringhem. It must here be stated, that it has unhappily be- 

 come custumary for some American authors to use the term 



